The use of optical storage disks for the recording of data has become a frequently used option. Data can be stored on a top and bottom side of the optical storage disk to increase the storage capacity of the disk. However, unlike magnetic storage disk readers, optical storage disk readers generally do not allow simultaneous reading of both sides of an optical storage disk. In those few devices allowing both sides of an optical disk to be read simultaneously, the device is generally a fixed media device capable of using only fixed disks.
In order to increase on-line data capacity and shorten the transfer time of data stored on an optical storage disk, it is desirable to store the data on both sides of a single disk (including the use of such methods as the "interleaving" of data). Thus, as optical readers simultaneously pass over both sides of the disk, the information stored thereon may be read in such a fashion as to allow twice the amount of data to be retrieved in the same time as normally required to read one side.
With optical storage disk readers, it is essential to keep the beam of light used to read and/or imprint the disk perpendicular to the surface thereof. The spot from the beam on the disk degrades (for optical reasons) as the beam varies from a perfectly perpendicular orientation. Thus, in a double side reader, it is necessary to provide a device that can maintain the vertical relationship between both sides of an optical disk and the optical readers associated therewith.
One attempt to utilize both sides of an optical storage disk is disclosed in U.S. Pat. No. 4,387,452, to Bricot, et al., June 7, 1983. The Bricot patent discloses the use of optical readers/recorders capable of simultaneously reading/recording on oppositely facing sides of two separate optical storage disks. The Bricot invention requires the use of an intricate optical device capable of adjustably focusing on the surface of the disk. A plurality of optical storage disks are stacked with the optical reading/recording devices between the disks for reading of the opposite faces The Bricot method requires the use of two separate disks for the utilization of the space on two sides of a disk. Therefore, it is necessary to record the data on sets of disks rather than on a single disk.
Another device designed to attempt to read both sides of an optical storage disk is disclosed in U.S. Pat. No. 4,271,489, to Siryj et al., June 2, 1981, and U.S. Pat. No. 4,286,790, to Siryj et al., Sep. 1, 1981. The Siryj devices use optics such as are disclosed in U.S. Pat. No. 4,097,895, to Spong, June 27, 1978, and U.S. Pat. No. 4,300,226, to Barnette et al., Nov. 10, 1981. The optics in Barnette and Spong are capable of focusing on the surface of a disk which has discontinuities thereon from varying thicknesses of the surface coating material in the range of 3 mils (0.076 mm). Spong and Barnette do not take into consideration and are not capable of adjusting to disks having a thickness that varies beyond the minute surface discontinuities.
Unfortunately, optical storage disks can typically vary in thickness from approximately 2.75 mm to approximately 3.50 mm. These thickness variations are due to the disk construction by various manufacturers and the coatings which are applied thereto for the recording of data. Thus, in order to properly read both sides of a single disk which varies in thickness, it is necessary to have optical devices capable of adjustably focusing on each surface while maintaining a perpendicular relationship to both sides. Heretofore, it has been too difficult and/or expensive to provide a device capable of sufficient focusing adjustment to compensate for the varying thicknesses of optical storage disks. Thus, there is a need for a method and apparatus capable of simultaneously focusing on both sides of a single optical storage disk of varying thickness for simultaneous reading of the two sides.